A solid polymer fuel cell is a system for taking out electric power by using, as a fuel, pure hydrogen, hydrogen gas obtained by modifying alcohol, etc. and electrochemically controlling the reaction between the hydrogen and the oxygen in the air. It enables a compact configuration to be achieved. Development work is underway for application for electric vehicles etc.
The configuration of a typical solid polymer fuel cell is shown in FIG. 1. The basic principle of a solid polymer fuel cell 1 is as follows: That is, in a solid polymer fuel cell 1, the fuel of hydrogen gas (H2) 8 is supplied from the anode side and passes through the gas diffusion layer of the carbon paper 4 and catalyst electrode part 3 to form hydrogen ions (H+) which in turn pass through the electrolyte of the solid polymer membrane 2 whereby, at the cathode side catalyst electrode part 3, hydrogen ions (H+) and oxygen (O2) in the air 9 supplied from the cathode side undergo an oxidation reaction (2H++2e−+1/2O2→H2O) and water (H2O) is formed. At the time of this oxidation reaction, the electrons 10 formed at the anode side catalyst electrode part 3 flow through the carbon paper 4 from the anode side separator 6 to the cathode side separator 7 whereby current and voltage is generated across the electrodes.
The solid polymer membrane 2 has an electrolyte with a strong acidity fixed in the membrane and functions as an electrolyte passing hydrogen ions (H+) by control of the dew point inside the cell.
The component member separator 5 of the solid polymer fuel cell 1 has the role of separating the two types of reaction gases, that is, the cathode side air 9 and the anode side hydrogen gas 8, and providing flow paths for supplying these reaction gases and the role of discharging the water produced by the reaction from the cathode side. Further, in general, the solid polymer fuel cell 1 uses a solid polymer member made of an electrolyte exhibiting a strong acidity. Due to the reaction, it operates at a temperature of about 150° C. or less and generates water. For this reason, the separator 5 for a solid polymer fuel cell is required to have, as material properties, corrosion resistance and durability and is required to have good electroconductivity for efficient conduction of current through the carbon paper 4 and low contact resistance with carbon paper.
In the past, as the material for the separator for a solid polymer fuel cell, much use has been made of carbon-based materials. However, separators made of carbon-based materials cannot be made thin due to problems of brittleness and therefore obstruct increased compactness. In recent years, breakage-resistant separators made of carbon-based materials have also been developed, but they are expensive in cost, so are disadvantageous economically.
On the other hand, separators using metal materials are free from problems of brittleness compared with carbon-based materials, so in particular enable increased compactness and lower cost of solid polymer fuel cell systems. Therefore, many separators using titanium and other metal materials superior in corrosion resistance have been developed and proposed. However, separators made of pure titanium or titanium alloy become larger in contact resistance with the carbon paper due to the passivation film formed on the surfaces during power generation, so had the problem of greatly reducing the energy efficiency of the fuel cells.
For this reason, numerous methods for reducing the contact resistance between member surfaces and carbon paper have been proposed for titanium-made separators in the past.
For example, separator materials for fuel cell use which cause a precious metal or precious metal alloy to deposit on the surface of a titanium material or form a film there by the sputtering method or PVD method so as to lower the contact resistance with the carbon paper (that is, to raise the electroconductivity) have been proposed (see PLTs 1, 2, 3, and 4). Further, a titanium material for a fuel cell which uses a titanium alloy to which a precious metal has been added and causes the precious metal element to precipitate on the titanium alloy surface so as to lower the contact resistance has also been proposed (see PLT 5).
However, these methods require the formation of expensive precious metal layers or precious metal particles on the surface of the titanium material, so have the problem of increasing the manufacturing costs of the separators.
On the other hand, to reduce the contact resistance between the surface of the titanium material of a separator and carbon paper without using an expensive precious metal, the method of shot blasting etc. Cr2N, CrSi2, VB, V8C7, VN, TaN, TaC, WC, WNb, or other electroconductive compound particles containing metal elements other than Ti on to the titanium material surface has also been proposed (see PLT 4). However, at the time of use of the fuel cell, metal ions are eluted from these electroconductive compounds to the MEA (assembly of the solid polymer type electrolyte member and electrode) thereby causing the electromotive force to fall and the power generation ability to otherwise decline in some cases. Further, from the viewpoint of recycling the separator material, when remelting a titanium material on which electroconductive compound particles are deposited in large amounts, the elements contained in the electroconductive compound will affect the mechanical properties of the titanium and end up impairing the workability etc.
PLT 6, while not limited to a titanium material for a separator, discloses to electrolytically pickle a titanium material on the surface of which a layer containing titanium carbides and/or nitrides is formed in an acidic aqueous solution or a neutral aqueous solution containing an acidifying agent and to use an acidic aqueous solution comprised of a nitric acid aqueous solution (1 to 10 wt %) and an acidifying agent comprised of Cr6+ ions. Note that, this electrolytic pickling is based on electrolysis using titanium as an anode (anodic electrolysis). However, the contact resistance before and after power generation, important as a required property of a separator, is not described.
In PLT 6, ESCA (same method as X-ray photoelectron spectroscopy (XPS)) at the surface of Example 1 is shown in FIG. 2. Except for contamination, no clear peak other than TiO2 (near 459 eV) is detected. That is, no peaks are detected at the spectral energy ranges showing the presence of TiO and metal Ti (respectively 454.2 to 455.1 ev and 453.7 to 453.9 eV).